1. Field of the Invention
This invention relates to a method for the manufacture of printed circuit boards and, in particular to improving the method by eliminating the need for a first electroless flash metal plating of the through holes prior to the use of protective coatings such as plating resists and soldermasks.
2. Description of Related Art
The metal plating of plastics and other nonconductive (dielectric) substrates is well-known in the art and the following description will be directed for convenience to plastics, especially epoxy, and the preparation of printed circuit boards.
The preparation of printed circuit boards requires the plating of conductive metal layers, usually copper, onto the plastic substrate of the board. These boards vary in design and may have a copper layer on each surface of the epoxy (two-sided boards) or they can be multi-layer boards which have a plurality of inter-leaved parallel planar copper and epoxy layers. In both types, through-holes are drilled in the board and metal plated to facilitate connection between the circuits on the copper layers.
There are basically two methods for preparing the boards, the "substractive" method and the "additive" method with many variations in the process steps being employed depending on the type board desired. Regardless of the methods used, however, the need exists to apply and remove protective films on the board during the process.
For purposes of illustration the semi-additive method will be described although the invention is applicable to other methods of manufacture. Commencing with a commercially available copper clad laminate or multi-laminate made from, e.g., paper-epoxy or glass-epoxy material, a predesigned series of through-holes is formed in the board in any conventional manner, then preferably treated to desmear or etch the plastics prior to the metal plating. A swell and etch procedure is usually used to treat the board and in one process the board is first contacted with a solvent to swell the epoxy and enhance the effect of the subsequent oxidative, e.g., permanganate solution etching step. After etching, the board is water rinsed and then neutralized with a reductant to solubilize manganese residues by lowering their oxidation state. The reductant preferably contains an acid fluoride to dissolve glass fibers in the epoxy. Thereafter the board is immersed in a catalyst, such as a tin-palladium solution, which activates the surface of the epoxy for electroless copper plating. Following a water rinse, the laminate is immersed in an accelerator to activate the catalyst by freeing the metal palladium ions on the board. After a water rinse, the board is dried and is immersed in an electroless copper plating solution for a period of time sufficient to flash plate copper on the through-hole connections.
The board is then coated with a photoresist (usually with tenting (covering) of the through holes), imaged and the unwanted copper etched using ammonium persulfate or other etchant. The photoresist is removed using a solvent such as KOH and a soldermask applied to cover the desired circuits. Electroless copper in this case is then plated on the pads and through holes for about 12-5 hours to build the copper thickness up to about 1-1.5 mils.
Flash plating is necessary to permit plating after the imaging process since the activation systems have not been able to withstand this procedure. Also, the above procedure is time consuming and costly and it is preferred to improve the process by eliminating the need for the flash electroless copper plating of the board prior to application of the coatings such as an imaging film and the procedures associated therewith.
It is an object of the invention to provide an improved process for manufacturing printed circuit boards in which a flash coating of copper is not required on the through holes during the board making process.
A further object is to provide a printed circuit board made by the improved process.
Other objects and advantages will be apparent for the following description.